As generally known in the art, cerium oxide (CeO2) is a highly functional ceramic material that is widely used in catalysts, phosphors, cosmetics and polishing agents, and has recently been spotlighted as an abrasive for use in an STI (Shallow Trench Isolation) process of a semiconductor device, and as an optical glass polishing agent. Such cerium oxide is generally prepared by a liquid-phase process, in which cerium oxide powder is formed directly from a trivalent or tetravalent cerium precursor by adding a precipitant thereto, a solid-phase process, in which cerium oxide powder is formed by providing an intermediate product, such as cerium carbonate, and then performing a firing step, and the like.
Particularly, in the solid-phase process, cerium carbonate is widely used as an intermediate product of cerium oxide, and research on the types and shapes of cerium carbonate is being actively pursued because they have a great effect on the physical properties and shapes of cerium oxide.
Several examples of conventional technology for synthesizing cerium carbonate powder in a solution phase include 1) a method of preparing orthorhombic cerium carbonate powder by subjecting a cerium salt and urea to a precipitation reaction, 2) a method of preparing hexagonal cerium carbonate powder by subjecting a cerium salt and urea to a hydrothermal reaction, and 3) an attempt to adjust powder crystallinity depending on the type of a salt, reaction temperature, reaction time, and concentration of urea when cerium carbonate powder is prepared by subjecting cerium chloride, cerium sulfide, or cerium nitrate anhydrate and urea to a hydrothermal reaction, and so forth. However, no study has been conducted on adjusting powder crystallinity at relatively low precipitation temperature under atmospheric pressure.
The crystal structure of cerium carbonate varies according to preparation methods, and particularly may be divided into an orthorhombic structure, a hexagonal structure, and the like. As far as is known, the orthorhombic cerium carbonate can be prepared by an aqueous solution-based precipitation reaction, and the hexagonal cerium carbonate can be prepared by high-temperature high-pressure hydrothermal synthesis. However, as processes are scaled up, high pressure employed in the hydrothermal synthesis becomes more dangerous, which results in high equipment cost.
Also, the use of urea as a precipitant has several advantages such as a very stable reaction, the improvement in uniformity of a powder property due to uniform precipitation formation, etc. However, in preparation of cerium carbonate under atmospheric pressure by an aqueous solution-based precipitation reaction, when urea is used as a precipitant, it may be difficult to control the crystal structure of cerium carbonate.